The first external fixator was developed for reducing and maintaining patellar fractures. Since that time, various fixators have been invented for splinting various bone fractures. Most of these fixators have common features. In particular, they rely on transcutancous pins or screws secured in the bone on either side of the fracture site. An external apparatus is attached to the pins to allow their relative positions to be adjusted, thus enabling a surgeon to reestablish alignment of the bone pieces at a fracture site. Once the bone is properly set, the articulations in the fixator may be locked into place to maintain the chosen alignment.
Principal variations among the many fixator designs include the number of degrees of freedom permitted the wrist and the relative independence of each articulation, both mechanical and geometric. The first external fixator, for instance, was adjustable only in length and squeezed the fracture together by gripping opposed ends of the patella. Fixators designed to repair central fractures of the long bones typically have relatively few articulations or degrees of freedom. In contrast, fixators adapted to treat fractures of bones in joint regions must permit articulation through many more degrees of freedom. Where there is insufficient room to place pins in the bone fragment between the fracture and the joint, bone alignment must be established using pins placed in a bone on the side of the joint opposite the fracture. Such treatment of fractures near joints such as the wrist, which can rotate, flex and abduct, requires that a fixator permit some movement through the range of motion of the joint in a manner allowing a surgeon to establish proper fracture alignment by using forces transmitted through the joint.
Modem fixators permit articulation by various methods. Probably the most common articulation is provided by a ball joint. A ball joint provides one rotational and two pivotal degrees of freedom. A single setscrew or other locking mechanism can fix all three degrees of freedom simultaneously. The disadvantage of this method of articulation is that it is not possible to loosen the joint to permit motion in only one of the possible degrees of freedom. Thus, a surgeon cannot loosen the ball joint slightly to pivot it a small amount in one direction without the possibility of introducing changes affecting the other pivot and rotation settings.
In order to overcome this limitation, some fixators eliminate ball joints and rely instead on a combination of independent articulations to provide the necessary freedom of movement. The benefit of such a system is that each degree of freedom is mechanically independent of every other degree of freedom. A surgeon may thus adjust the position of a single articulation in the fixator without affecting the settings of other articulations.
Most fixators also include some type of extensible/contractible articulation to permit the longitudinal spacing between the pins on opposite sides of the fracture to be controlled. This type of translational freedom can be used to accommodate individuals of varying size, as well as to distract the fracture, if necessary.
The wrist joint permits the hand to move in three degrees of freedom relative to the forearm. First, the hand can move in supination and pronation, i.e., the rotation about the longitudinal axis of the forearm. Second, the hand can move in adduction and abduction, i.e., lateral flexion, or pivoting about an axis perpendicular to the plane of the palm. The last type of mobility of the hand is dorsal-palmar flexion, which is the pivotal motion about an axis in the plane of the palm and perpendicular to the longitudinal axis of the forearm.
Dynamic fixators allow for some movement while affixed to a joint region, such as a lower arm, wrist and hand, yet maintain sufficient alignment of the fracture while it heals. Such prescribed movement can reduce and assist in recovery from stiffness associated with immobilization of the fracture.
Fixators have typically been designed from a purely functional viewpoint. Fixator design and construction has generally been bulky and unsightly, and generally increase the effective dimensions of the wearer's hand, wrist and arm to an awkward degree. Further, these fixators have typically been far more robustly designed than is necessary. The size and weight of these fixators thus add an undesirably bulky and clumsy addition to the wearer's arm and hand.
One of the more common fractures requiring a fixator for proper treatment is a fracture of the distal radius, or Colles fracture. This type of fracture usually results from a fall upon an outstretched hand. The fracture line is usually quite close to the distal head of the radius and sometimes the head is comminuted. Because of the lack of space and the number of tendons and nerves in the area, it is difficult to mount pins in the radius on the distal side of the fracture. Therefore, such fractures are typically reduced using a pair of pins set in the index, or second metacarpal and a pair of pins set in the radius on the proximal side of the fracture. In order to avoid damage to tendons and nerves, the radial pins are usually set in the third quarter of the radius, i.e., the proximal half of the distal half of the radius. With the pins are set on opposite sides of the wrist joint, the fixator must be sufficiently articulate to reduce the fracture using forces transmitted through the wrist joint.